A discharge lamp such as a metal halide lamp used in a vehicle headlight is lit in the following manner. A high-voltage pulse (e.g., several tens kV) for prompting a dielectric breakdown between electrodes is first applied, and a discharge arc is struck between the electrodes and the portion between the electrodes is brought into conduction. Next, relatively large electric power is supplied to increase light emission intensity quickly. Thereafter, a voltage (lamp voltage) between the electrodes of the discharge lamp increases as the light emission intensity by vaporization of metal sealed inside a tube increases, so that the supplied electric power gradually is decreased according to an increase in this lamp voltage. In this manner, the light emission intensity of the discharge lamp quickly converges to a predetermined intensity while preventing an overshoot.
At present, a trace of mercury is sealed in the discharge lamp such as the metal halide lamp. However, a (mercury-free) discharge lamp without containing mercury is being developed to prevent environmental contamination at the time of disposal. FIG. 14(a) is a graph showing a typical example of changes (from a start of lighting) in luminous flux (graph G10), lamp voltage (graph G11) and supply electric power (graph G12) in a conventional discharge lamp in which mercury is sealed. Also, FIG. 14(b) is a graph showing a typical example of changes (from a start of lighting) in luminous flux (graph G13), lamp voltage (graph G14) and supply electric power (graph G15) in a mercury-free discharge lamp.
In the conventional discharge lamp in which mercury is sealed, a lamp voltage immediately after starting lighting is about 27 V and gradually increases to about 85 V with an increase in light emission intensity as shown in FIG. 14(a). A lighting circuit decreases the supply electric power from about 70 W to about 35 W according to a change (amount of change 58 V) in this lamp voltage. On the other hand, in the mercury-free discharge lamp, a lamp voltage immediately after starting lighting is equal to that of the discharge lamp with mercury (about 27 V) and the lamp voltage increases to about 45 V with an increase in light emission intensity, but the amount of change (18 V) is smaller than that of the discharge lamp with mercury as shown in FIG. 14 (b). Also, a lamp voltage value immediately after starting lighting or the amount of change in the lamp voltage with an increase in light emission intensity has variations depending on secular change or individual difference. When the amount of change in the lamp voltage is small, an influence of the variations by secular change or individual difference becomes relatively great, so that it becomes difficult to speedily converge the light emission intensity while preventing an overshoot in a method for controlling the supply electric power according to the lamp voltage value.
To address the problem of electric power supply to the mercury-free discharge lamp as described above, a discharge lamp apparatus described, for example, in Japanese Patent Reference JP-A-2003-338390, is intended to reduce an influence on electric power control by variations in lamp voltage of individual discharge lamps by storing a lamp voltage (lamp initial voltage) immediately after a start of lighting and controlling supply electric power based on the amount of change in the lamp voltage from this lamp initial voltage.
However, the discharge lamp apparatus described in JP-A-2003-338390 can present the following problem. As described above, a high-voltage pulse for prompting a dielectric breakdown between electrodes is first applied in the case of lighting a discharge lamp. A lamp voltage immediately after a start of lighting is influenced by this high-voltage pulse and becomes unstable, so that in a method using the lamp voltage immediately after the start of lighting as a lamp initial voltage, a value of the stored lamp initial voltage varies every operation and the amount of change in the calculated lamp voltage also varies every operation. Therefore, in the discharge lamp apparatus described in JP-A-2003-338390, it is difficult to control the supply of electric power with good reproducibility.